A spark plug has been heretofore used for igniting an internal combustion engine. In the spark plug, a spark discharge gap is generally formed in such a manner that a ground electrode is welded to a front end portion of a metal shell for holding an insulator including a center electrode inserted therein so that the other end portion of the ground electrode faces a front end portion of the center electrode. Spark discharge is performed between the center electrode and the ground electrode. To improve resistance to spark abrasion, a noble metal tip is further formed in a region of each of the center electrode and the ground electrode between which the spark discharge gap is formed.
Incidentally, as a method of joining the noble metal tip to the center electrode of the spark plug, a recess (small-diameter portion) is provided in the front end portion of the center electrode so that the tip (discharge noble metal electrode) is resistance-welded to the recess and then the whole circumference of a side surface portion of the tip is laser-welded to the front end portion of the center electrode to thereby improve bonding strength between the tip and the front end portion of the center electrode (e.g. see Patent Document 1).
[Patent Document 1] Japanese Patent Laid-Open 22155/1995